Note: I copied this directly from weddingchannel.com.

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Semi-formal: This is, unfortunately, one of the trickiest types of dress code and one of the most common. In general, if the wedding is being held after 5 p.m., guests should assume semi-formal or formal attire is required, unless otherwise stated on the invitation. Even if it's an outdoor evening wedding, guests may still be required to wear semi-formal attire. For women, this generally means a nice cocktail dress and heels; for men, this means a suit and tie. Guests can always check with each other or ask about what the bridal party is wearing if they're still unsure.